1. Field of the Invention
The present invention relates to an ultrasonic color Doppler imaging system, and more particularly, to a color imaging display method and apparatus which can display a high quality of a color image in which a flash artifact which is a kind of a clutter signal included in an ultrasonic signal is effectively removed.
2. Description of the Related Art
A general ultrasonic testing apparatus emits an ultrasonic signal to an object to be tested, receives an ultrasonic signal reflected and returned from a discontinuous surface of the object, and then converts the received ultrasonic signal into an electrical signal and outputs the electrical signal to an imaging apparatus, in order to test the internal state of the object. The ultrasonic testing apparatus is widely used for medical diagnosis, non-destructive testing and underwater photographic search.
A Doppler diagnostic apparatus which is a kind of a diagnostic apparatus adopting an ultrasonic signal displays a frequency shift amount together with a signal scattering intensity as an image, to thereby estimate the dynamic functions in the living body. In particular, a color doppler imaging system which can display a color ultrasonic image demodulates a received signal and then digitizes the received signal to process it in digital form, thereby displaying a blood stream flowing in the heart or main blood vessel as a two-dimensional image in real-time. Such a color doppler imaging system can display a tomographic image and blood stream information at the same time, in which the tomographic image is displayed in black and white and the blood stream information is displayed in color, in order to discern the tomographic image and the blood stream information from each other. In this case, a blood stream flowing in the proceeding direction of a scanned ultrasonic beam is displayed as a warm color of red color series and the other blood stream flowing in the opposing direction to the scanned ultrasonic beam proceeding direction is displayed as a cool color of blue color series, in order to display the information on the blood stream with more accuracy.
Meanwhile, the doppler signal demodulated by the above color doppler imaging system contains a low-frequency doppler signal due to movement of the cardiac wall or the tissue of the human body. The low-frequency doppler signal that is called a clutter signal impedes an accurate detection of the blood stream information. Thus, to accurately detect the blood stream information, the clutter signal should be properly removed. The clutter signal due to the cardiac wall or tissue movement contains a very high-level component in general, and has several hundred times an amplitude as large as that of the doppler signal based on the blood stream. The large clutter signal appears frequently when the human tissue moves severely as in the case of the movement of the blood vessel wall or the heart pulse, or when the patient breathes during diagnosis or the doctor moves a probe for diagnosis. The pixels including the clutter signal look like as if a color box is filled with a color image of a dim color representing a slow velocity. The clutter signal is called a flash artifact since it looks like a lightening in real-time image.
Various methods have been proposed in the conventional art in order to remove the flash artifact. First, there is a method of enhancing the cut-off frequency of a clutter filter. The cut-off frequency enhancement method can remove the flash artifact having a slightly fast velocity but cannot remove that having a largely fast velocity due to the limit of the filtering level based on the stop-band characteristic of an infinite impulse response (IIR) filter. Second, there is a method of adaptively varying a cut-off frequency of the clutter filter disclosed in U.S. Pat. No. 5,664,575 entitled xe2x80x9cUltrasonic Doppler Imager Having Adaptive Tissue Rejection Filter.xe2x80x9d This method can reduce the flash artifact if a cut-off frequency becomes high as in the above first method. However, this method has a demerit in which useful doppler information disappears. Third, there is a method of using an inter-image frame filter disclosed in U.S. Pat. No. 5,722,412 entitled xe2x80x9cHand Held Ultrasonic Diagnostic Instrument.xe2x80x9d This method uses a min-max filter and a non-linear filter according to the relationship of a successively calculated color framing and can remove the flash artifact irrespective of the velocity and power of the flash artifact. However, the frame which has been displayed for non-linear filtering between successive frames is delayed in comparison with an actually scanned frame.
To solve the above problems, it is an object of the present invention to provide a method and apparatus for displaying a high quality of color image in a color doppler imaging system, in which a doppler signal where an ultrasonic signal is filtered and a clutter signal where the ultrasonic signal is not filtered are frequency-estimated, and information obtained through the frequency estimation is mixed in order to remove flash artifact.
To accomplish the above object of the present invention, there is provided a method of displaying a color image in a color doppler imaging system, the color image displaying method comprising the acts of: (a) filtering a clutter signal from an ultrasonic signal reflected from an object; (b) analyzing a respective frequency spectrum of the ultrasonic signal and an ultrasonic doppler signal filtered in act (a), to thereby calculate clutter information and doppler information; (c) determining whether a flash artifact exists based on the clutter information and the doppler information calculated in act (b); and (d) determining whether a color image of a corresponding pixel is displayed according to the determination result of act (c).
According to the other aspect of the present invention, there is provided a color image displaying apparatus for displaying a color image for removing a flash artifact in a color doppler imaging system, the color image displaying apparatus comprising: a clutter filter for filtering an ultrasonic signal reflected from an object and outputting a doppler signal obtained by removing a clutter signal from the ultrasonic signal; a post processor for analyzing a respective frequency spectrum with respect to input and output signals of the clutter filter, determining whether a flash artifact exists using information obtained from the analyzed result, and outputting pixel display information in such a manner that a pixel to be determined as the flash artifact is not displayed; and a display for displaying an ultrasonic color doppler image according to the pixel display information of the post processor.